This invention generally relates to interferometry and more particularly to interferometric apparatus and methods by which highly precise metrology is practiced.
In highly precise metrology, it is required to understand and compensate for changes in the direction of propagation of various individual optical beams in which information is encoded about distances and/or angles under measurement. Changes in the direction of propagation of such beams or in their angular relationships with respect to one another, or some reference, are often introduced by pitch, yaw, and roll of various optical components in the interferometer as, for example, those that might be experienced by a quickly stewing planar measurement mirror in a plane mirror interferometer, or the like. If such effects are not compensated, it is often not possible to achieve the desired precision required of important industrial applications such as those practiced in the fabrication of semiconductor devices.
In addition, where interferometers have dynamic elements that may be used to control the direction of propagating beams, some means for measuring errors in beam direction and a measurement of a change in an angle of an object is needed to provide feedback signals for controlling such elements.
Consequently, it is a primary object of the present invention to provide apparatus and methods for measuring the direction of propagation among one or more optical beams.
It is another object of this invention to provide apparatus and methods by which changes in the direction of propagation and/or angular relationships among one or more optical beams may be measured with interferometric precision.
Other objects of the invention will in part appear hereinafter and will in part be obvious when reading the following detailed description in connection with the accompanying drawings.